Electrical properties of electrical devices may deteriorate over time without the protection of an oxygen or a moisture barrier sealant. In particular, a polymeric positive temperature coefficient (PPTC) device may consist of air sensitive and low resistivity nickel particles dispersed in a partially crystalline polymer matrix, and over time, ambient oxygen may permeate into amorphous regions of composite and oxidize the surface of the nickel particles. As a result, after a trip event or thermal cycling, the electrical pathway that is reestablished consists of surface oxidized nickel particles in contact with each other. The multitude of oxidized contact points results in an increase in electrical resistance. This increase results in devices that are “out of spec” and unacceptable for use. To prevent oxidation, electrical devices may be externally coated with a two-part epoxy-based oxygen barrier composition.
While the current two-part compositions may provide an effective oxygen and moisture barrier for certain applications such as protection of battery strap PPTC devices, they are somewhat limited in their scope of use and application. Some disadvantages of the two-part compositions include: a short pot life (i.e., working life), a substantial fraction of organic solvent (e.g., 20%), halogen contaminants, and a restriction of application to thin coatings on the outside of larger form factor PPTC devices. These coatings are also typically applied in a multi-step overcoating process. These solvent-borne formulations are also difficult to implement in underfill or gap fill type applications because of solvent trapping issues in the interstitial spaces of the laminate or other structures containing the active devices.
Therefore, there is a need for improved oxygen barrier compositions for electrical devices with improved pot life, low or no volatile organic compounds (“VOCs”), as well as reduced levels of halogen contaminants, such as chlorine and bromine. There is also a need to prevent oxidation of smaller form factor PPTC devices, such as surface mount devices (SMDs), which are more susceptible to nickel filler oxidation because they have a greater surface area to volume ratio, and are difficult to coat using standard methods. Finally, there is also a need for control over the degree of cure of the resin system during various stages of the device manufacture process, especially in terms of SMD PPTC device manufacture.